Titanium oxide photocatalytic materials, which are capable of decomposing hazardous substances in the atmosphere or water simply by being irradiated with light, are attracting worldwide attention, and are expected to be applied to various fields.
Titanium oxides have rutile, anatase, and like crystal structures. Of these, anatase-type titanium oxide is known for its high photocatalytic characteristic.
Most titanium oxide photocatalysts are in the form of fine particles with a diameter of several nanometers to several tens of nanometers so as to ensure a sufficient surface area. To prepare a practical photocatalytic material, the titanium oxide photocatalysts are applied onto a base substrate to form a coating layer using a binder component.
In this method using a binder, most of the titanium oxide fine particles are immersed in the binder, leaving only a trace amount of titanium oxide exposed to the surface. Thus, a trace amount of titanium oxide is conducive to the actual reaction, and the photocatalytic activity inevitably decreases.
In order to obtain a photocatalytic material that is suitable for industrial production and that ensures formation of a sufficient amount of anatase-type titanium oxide, the inventors of the present invention previously invented a method of forming a titanium nitride on a surface of a titanium metal or a titanium alloy, and subjecting the metal or the alloy to anodization (Japanese Unexamined Patent Application Publication No. 2005-240139).
However, although this invention succeeded in improving the activity of a photocatalytic material compared with those of hitherto-known photocatalytic materials, its effect was still insufficient in fields requiring quick decomposition of hazardous substances in gas or liquid phases.
On the other hand, Japanese Unexamined Patent Application Publication No. H06-142440 discloses a hazardous substance decomposition and/or removal technique that relies on the oxidization properties of hydrogen peroxides. In this technique, 0.1 to 10 weight %, more preferably 1 to 5 weight % of hydrogen peroxide, alkali content hydrogen peroxide, or the like is incorporated in a breathable porous body, such as silica gels, zeolite, or activated carbon, thereby oxidizing hazardous gas due to the oxidization properties of the hydrogen peroxide. However, because this method performs decomposition and/or removal by relying only on the oxidization properties of hydrogen peroxide, it is necessary to use highly concentrated hydrogen peroxide, which harms the environment with its strong toxicity and corrosive properties.
In addition, there has been research into a technique for decomposing and/or removing hazardous substances by causing a photocatalyst to coexist with hydrogen peroxide.
Japanese Unexamined Patent Application Publication No. 2000-70968 discloses a technique for decomposing and/or removing hazardous substances by using a photocatalyst obtained by a hitherto-known coating method, ozone, and hydrogen peroxide. However, in this technique, if only hydrogen peroxide is added, without adding ozone, the decomposition and/or removal effect of the photocatalyst is insufficient; therefore, using both ozone and hydrogen peroxide with a photocatalyst is indispensable in this method.
Further, Japanese Unexamined Patent Application Publication No. 2006-35140 discloses a technique for removing contaminants in water such as fenitrothion by irradiating the water with an ultrasonic wave at a frequency of 28 to 45 kHz, as well as using photocatalytic particles and hydrogen peroxide.
Additionally, Japanese Unexamined Patent Application Publication No. 2010-22958 discloses a technique for decomposing persistent agricultural chemical components using a photocatalyst. In this technique, contaminants are decomposed and/or removed by using 10 ppm to 50 ppm of ozone, 5 ppm to 30 ppm of oxygen, and 200 ppm to 2500 ppm of hydrogen peroxide, after adjusting the pH to 6 or more.
The large amount of sulfur oxides, nitrogen oxides, etc., generated by the combustion of fossil fuel in coal-fired plants or the like has always been problematic. These hazardous substances in gas phases cause acid rain. As a method for removing sulfur oxides, a method of reacting a limewater slurry with a sulfur oxide gas, and removing the resulting calcium sulfates has often been used. As a method for removing nitrogen oxides, ammonia catalytic reduction, which reduces nitrogen oxides using ammonia, has been often used.
As explained above, although various photocatalysis technologies using a combination of a photocatalyst and hydrogen peroxide have been published, none of those technologies ensure a sufficient effect by combining only hydrogen peroxide and a photocatalyst, unless using ozone, ultrasonic wave, or the like together with hydrogen peroxide and a photocatalyst.
Further, the limewater slurry method and the ammonia catalytic reduction for removing a large amount of sulfur oxides, nitrogen oxides, etc., resulting from the combustion of fossil fuel in coal-fired plants or the like also have drawbacks such as high cost, and the toxicity of ammonia.